1. Field of the Invention
This invention relates to conformal protective coatings for electronic devices and, in particular, to a copolymer which is a reaction product of a tetracarboxylic acid dianhydride, a diamine and a di(aminoalkyl)-siloxane.
2. Description of the Prior Art
A semiconductor device which exhibits an undesirable resistance to reverse biasing breakdown often has been found to have detrimental leakage currents existing on selected surface areas during operation of the device. The leakage currents arise from the mobility of ions which are formed from ions of adsorbed molecules.
Heretofore, prior art methods to correct such deficiencies provided a means for protecting selected surface areas of electrical devices, in particular, exposed end portions of P-N junctions of a semiconductor device, with an electrically insulating material such, for example, as silicon oxide, silicon nitride, aluminum nitride and the like. Such materials are provided as a thin layer of a coating material on the selected surface area and have virtually no resistance to mechanical abrasion and require relatively expensive processing techniques and equipment. In many instances a second, and thicker, layer of coating material is disposed on the layer of electrically insulating material to provide a suitable abrasive protective means therefor. Silicone varnishes, rubbers, resins and the like are employed as the overcoating material but often lack suitable physical characteristics. Often these silicone materials require two components which must be mixed together, a catalyst to aid in curing, extensive curing times up to 24 hours and more, and chelating agents admixed therein.
Improvements have been made such, for example, as provided by Shaw in his U.S. Pat. No. 3,615,913. However, the improved material lacks desirable adhesion and electrical properties required for some electronic device applications.
In a series of U.S. patents including U.S. Pat. Nos. 3,325,450 and 3,740,305, Holub teaches a Ter-Polymer System consisting of a Diamino Siloxane, an organic diamine and a Tetracarboxylic Acid Dianhydride are disclosed. In these patents the combinations of the Diamino Siloxanes with the organic diamine varied between the two extremes in conjunction with molar quantities of Tetracarboxylic Acid Dianhydride. It is not apparent from the teachings of these patents that any specific combination of components would be more effective coating materials over and beyond their direct relationship to either the 100% polyimide or 100% polysiloxane terminal component.
It is an object of this invention to provide a new and improved conformal coating suitable for use on electronic components.
Another object of this invention is to provide a polyimide-siloxane copolymer material for use as a conformal coating on electronic components wherein the copolymer contains from 18 to 45 mole percent of the silicone diamine.
Another object of this invention is to provide a copolymer which is a reaction product of a tetracarboxylic acid dianhydride, a diamine and a di(aminoalkyl) siloxane.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.